Protein kinases are now recognized as key regulatory molecules participating in myriad biochemical pathways. As catalysts, these enzymes are capable of amplifying intra- or extracellular signals. By virtue of their broad yet limited substrate specificity, they are able to regulate more than one substrate protein at a time, coordinating intracellular metabolism. Understanding how protein kinases regulate cell processes is a fundamental problem in biology. Casein kinase-l is a protein that has been identified in virtually every eukaryotic cell in which it has been sought, suggesting that it performs an important biological function. In spite of this observation, little is known about what that function may be. The goal of this proposal is to exploit the advantages of the yeast Saccharomyces cerevisiae as a biological system to study the cell biology of casein kinase-l in detail. Specifically, the protein itself will be studied by determining its intracellular localization, the nature and degree of its post- translational modification in vivo, and levels of gene expression during the cell cycle. The gene encoding casein kinase-l will be cloned, revealing the primary structure of the protein, and leading to a genetic analysis of casein kinase-l function in yeast cells. This work will be extended by developing novel strategies for identifying biologically relevant substrates of casein kinase-l and for elucidating the consensus amino acid sequence required for casein kinase-l recognition of substrates. The methods developed to study casein kinase- l can be applied to the study of any protein kinase.